Hydrocarbons such as oil, natural gas, etc., may be obtained from a subterranean geologic formation, e.g., a reservoir, by drilling a well that penetrates the hydrocarbon-bearing formation. This provides a partial flowpath for the hydrocarbons to reach the surface. In order for oil to be produced, that is travel from the formation to the well bore (and ultimately to the surface), there must be a sufficiently unimpeded flowpath from the formation to the well bore. Unobstructed flow through the formation rock (e.g., sandstone, carbonates) is possible when rock pores of sufficient size and number are present for the oil to move through the formation.
However, as is becoming more generally known, greater effort and more varied approaches must be undertaken to produce hydrocarbons since the relatively easier to produce subterranean formations have generally been found. Thus, the oil and gas industry is looking at producing hydrocarbons from subterranean formations where recovering the hydrocarbons is more difficult and requires many steps, including the introduction and placement of various components, additives and agents at relatively precise locations downhole.
One such more complicated process involves hydraulically fracturing the subterranean formation—literally breaking or fracturing a portion of the strata surrounding the wellbore. The development of suitable fracturing fluids to provide the necessary hydraulic force is a complex art because the fluids must simultaneously meet a number of conditions. For example, they must be stable at high temperatures and/or high pump rates and high shear rates which can cause the fluids to degrade and prematurely settle out the proppant before the fracturing operation is complete. Various fluids have been developed, but most commercially used fracturing fluids are aqueous based liquids which have either been gelled or foamed. When the fluids are gelled, typically a polymeric gelling agent, such as a solvatable polysaccharide is used, which may or may not be crosslinked. The thickened or gelled fluid helps keep the proppants within the fluid during the fracturing operation.
Aqueous fluids gelled with viscoelastic surfactants (VESs) are also known in the art. VES-gelled fluids have been widely used as gravel-packing, frac-packing and fracturing fluids because they exhibit excellent rheological properties. VES fluids are non-filter cake-building fluids.
In some cases aqueous fluids are gelled with both a VES and a hydratable polymer to gain some of the advantages of both gelled fluid types. While such hybrid fluids may be “broken” or have their viscosities reduced by a separate conventional VES-gel breaker and a separate polymer gel breaker, it would be desirable if one breaker system could be devised for such hybrid fluids.